The present invention relates to organic nonlinear optical crystal, a method of making same, and the materials from which they are made.
Organic nonlinear optical crystals are important for many applications. In particular, organic second order optical materials have the potential for a variety of applications in signal processing and other areas of optoelectronics and photonics. Many of these applications require the materials to be in the form of single crystal films. The reason for this is that unless the molecules are organized as, for example, a crystal, the second order optical effect disappears. The thin film form is essential for device applications in optics that require low-loss optical propagation through the film. Therefore, significant attention has been paid by researchers in this field to establish a systematic method for preparation of single crystal thin films of organic second order optical materials. However, efforts to create thin single, crystal films from second order optical organics have largely been unsuccessful.
The major difference between the crystal growth mechanism of an organic material and an inorganic material is that the growth subunits in the case of inorganic materials are atoms whereas the growth subunits of organic systems are molecules. The atomic subunits being spherical in shape, and single-membered unitsxe2x80x94are easy to arrange within a desired lattice structure. The subunits of organic systems being molecular, contain large numbers of atoms linked together by covalent bonds (e.g., a specific diacetylene molecule (4-BCMU [diacetylene-(4-butoxycarbonyl)methylurethane)]) contains 76 atoms). Such multimembered units usually have complicated shapes and structures that are easily deformable to different conformational states. Therefore, organization of these organic molecules in a specific lattice structure is usually more difficult than that of inorganic materials.
By virtue of the simplicity of the growth mechanisms and shape of the growth subunits, inorganic materials are relatively easy to organize by the epitaxial method, known to those of ordinary skill in the art. Matching of the lattice parameters between a substrate and organizing material provides the required driving potential for the systematic growth. Because of spherical subunits (atoms), most inorganic systems possess cubic unit cells and within the inorganic family usually it is easy to find suitable substrates with appropriate lattice spacings for the epitaxial growth of one member on another. That is why the epitaxial method is so commonly and successfully used in thin film organization of inorganic systems.
Organic molecules (that are solid at room temperature) being large in size, the unit cell dimensions of organic crystals are much larger than that of inorganics (e.g., some diacetylene monomers such as poly(4-BCMU) have lattice spacings more than 3.0 nanometers (xe2x80x9cnmxe2x80x9d)). Because of the structural anisotropy of the molecules organic crystals more commonly possess monoclinic unit cells. If the epitaxial method is to be used it is necessary to obtain a substrate that has a similar molecular and crystal structure as the organizing material. Inorganic materials usually have cubic unit cells and lattice spacings of about 5 nm. Therefore, inorganic substrates are inappropriate for epitaxial growth of an organic material. Within the organic family it is difficult to find a material that can be grown as a large crystal with good optical quality surfaces and can be utilized as a substrate for the epitaxial growth of another organic material. Therefore, the epitaxial method has not been useful for thin film crystal growth of organic materials. Attempts of using epitaxial method have failed to produce large area (i e.,  greater than 1 mm2) single crystal films of organic materials. These findings show that the principal difficulty in the thin film organization of organics stems from the complicated structures of the molecules. It has not been fruitful to take methods that are specifically devised for inorganics and apply them to organics. For organic materials fundamentally novel approaches have to be identified that would take full advantage of the molecular nature of the subunits.
A method that is well known for providing one-dimensional ordering for specific organic molecules is the Langmuir-Blodget (L-B) method. The polar characteristics of amphiphilic molecules are effectively utilized in this scheme of organization, although the organization is limited to one dimension (perpendicular to the air-water interface) only. It would be desirable to have materials and a method that leads to the successful formation thin single crystal films (three dimensional ordering) with controlled orientation and thickness.
Thin film crystal growth was previously attempted using: (i) deposition from vapor phase, and (ii): slow cooling from melt in a cavity. However, the vapor phase method failed to produce large area samples (i.e.,  greater than 50 xcexcm) (see, Forrest et al. Appl. Phys. Lett., 68 1326 (1996); Burrows et al., J. Cryst. Growth, 156 91 (1995)). The melt growth method leads to attachment of the material to the cavity surface and the sample can not be used for characterization and application (see, Ledoux et al., Opt. Eng., 25 202 (1986). The melt growth of organic materials often leads to chemical decomposition of the material during the crystal growth. The molten material can become stuck to both substrates and therefore after the solidification step the substrates cannot be separated without damaging the film. In addition, melting usually leads to decomposition of these materials and the films that are produced are not single crystals, but polycrystalline. U.S. Pat. No. 5,412,144 to Stupp et al. describes a method using direct evaporation of solvent to form polycrystalline films having multiple domains. It does not disclose, teach or suggest the formation of single crystal films.
It would be desirable to have a reliable reproducible method for forming single crystal films of a usable size from second order organic materials. Such method would be adaptable for commercial manufacturing operations and industrial applications.
A novel method has been established for preparation of single crystal films of organic materials. Specifically, this method is applicable to organic second order optical materials which have a wide range of potential applications. This method produces films of excellent optical quality and with large nonlinearities suitable for device applications.
In accordance with a preferred embodiment of the present invention, a single crystal film can be produced from an organic second order optical material produced by a method comprising the steps of: (a) providing an organic second order optical material; (b) dissolving said optical material in a polar solvent so as to form a saturated solution at the temperature that is used for growth of said crystal, said solvent having a boiling point less than about 100xc2x0 C.; (c) providing at least two substrates; (d) forming hydrophilic surfaces of said at least two substrates; (e) drying said at least two substrates; (f) placing an effective amount of said solution of step b) on one of said at least two substrates; (g) contacting said substrate of step f) so as to form an assembly of substrate-solution-substrate; (h) moving at least one of said at least one substrates of step g) with respect to the other substrate so as to form a generally uniform layer of said solution between said at least two substrates; (i) placing said assembly of step h) is a closed chambers, said chamber being saturated with an effective amount of vapor of said solvent of step b; (j) evaporating at least a portion of said solvent in said chamber and said assembly over an extended period of time; and, (k) removing said assembly from said chamber such that single-crystal film forms on said at least one substrate upon continued evaporation of said solvent.
The present invention also provides single crystal films formed by the process described above.
Other objects, features, and advantages of the present invention will become apparent upon reading the following detailed description of embodiments of the invention, when taken in conjunction with the accompanying drawings and the appended claims.